The current XMM-Newton catalogue can be considered as one of the richest astrophysical resources available at ESAC. Therefore it was chosen as the main focus of the ARCHES project, and the base catalogue for which multi-wavelength products will be created.

The ARCHES project consists in three distinct but interconnected goals:

Goal 1: Enhanced XMM-Newton source catalogues

The most recent catalogue release is the 3XMM catalogue which contains about 350 000 unique sources and also benefits from a range of processing improvements. The scientific use of the XMM-Newton catalogue will benefit enormously from additional information on a wide range of data quality issues, completeness and reliability. There is also the opportunity to enhance the XMM-Newton catalogues with new parameters, for example the addition of long-term variability indicators and the results of automatic characterisation of the X-ray spectra of the brightest sources. Additional areas that will be explored include the use of external databases (e.g. Chandra) for improving data quality.

Alongside these catalogue enhancements, we will produce a detailed analysis of data quality, reliability and completeness issues and how these relate to detection parameters and flags that will enable users of the enhanced catalogue to accurately assess the impact of different catalogue selections for their science project Users will,for example, be able to select the ‘cleanest’ version of the enhanced catalogue by including only those sources with the best data quality and be able to know the reliability and completeness of this catalogue subset. More sophisticated users of the enhanced catalogue will be able to select catalogue subsets which accurately match the needs of their science project. These analysis results are in fact the most important element of the enhanced catalogue activity. The current publicly released catalogues already contain much of the relevant information, although often in a rather basic form, but what is lacking is the detailed information that allows the user to interpret and utilise the information effectively.

Goal 2: Creating scientifically validated multi-wavelength products

The Virtual Observatory (VO) offers multi-wavelength views on any region of the sky, and tools to construct spectral energy distributions and for cross-correlating large catalogues. Although the functionality of these VO tools is good, their scientific use still requires a careful check of the quality and relevance of the data entering the analysis, a step that is eventually left to the end user.

Our goal is to join in a common effort the scientific expertise of a leading group of astronomers and the skills of institutional partners deeply involved in data dissemination and in the development of the Virtual Observatory and of their protocols and tools to generate products with high scientific added value. More specifically, our aim is to produce best quality catalogue cross-identifications and spectral energy distributions for XMM-Newton sources extracted from the ESAC databases. The key points ensuring the quality of the end products are:

Catalogue selection. Not all catalogues have the same 'photometric' quality, nor the best calibration. It is therefore important to thoroughly assess the uncertainties on the final flux extracted from the archival catalogue and properly report them in the SED. The accuracy with which the position of the sources on the sky are known is another important parameter since this eventually determines the possibility to positively crossidentify or not, sources detected at different wavelengths.

Cross-identification. Making sure that the same astrophysical object was observed at widely different wavelengths is obviously a key issue. Several cross-correlation techniques have been developed in the last years that carry out a careful statistical treatment of the cross-matching of an arbitrary large number of catalogues. The various classes of astrophysical objects, such as stars, galaxies, etc, display spectral energy distributions with known properties and variance. If several candidates match with nearly equal probabilities based on positional constraints, comparing the resulting SEDs will help to identify the right counterpart. Our group has developed such algorithms and implemented them as software tools. In order to fulfill ARCHES’s goals, we need to extend the capabilities of our crossmatching tools in order to properly handle complex priors on SEDs while taking into account an arbitrary large number of catalogues.

Flux calibration. The usability of these SEDs is conditional on the accuracy of the flux calibration available in the various energy bands. This is not a trivial issue considering the number of observational steps entering in the final calibration and in general, good cross-calibration requires an excellent understanding of the instruments. One should also pay careful attention to the homogenisation between catalogues overlapping in wavelength so that variability can be quantified efficiently. We also plan to use theoretical spectra of stars, star forming regions, AGNs, X-ray emitting plasmas, etc. to allow easy comparison with the observed spectral energy distributions. This comparison will allow us to check the correctness of the assembled multi-wavelength data and detect departures from theoretical expectations that will shed new light on the object. Both observed and theoretical SEDs will be handled in VOSED. VOSED6 is a VO-tooldeveloped in the framework of the Spanish VO to facilitate the SED generation.

Archival catalogue:

Compiling the list of archival catalogues to query, following the criteria of quality and scientific relevance defined above is the focus of a dedicated work package. Some of the candidate astronomical archives that may be used to build SEDs are listed below.

ESAC archives

CDS archives

ESO public surveys: The European Southern Observatory, has executed several large area optical and near infrared surveys with the VST and VISTA telescope. The ESO public survey teams will publish source lists with excellent positional and photometric precision.

The use of Chandra serendipitious catalogue will also be investigated and whenever possible we will use the Chandra positions in the cross-matching process.

A large number of new astronomical surveys are planned for the forthcoming decade. On the ground, the ESO public surveys will likely be completed and extended after the end of our project. In a similar manner, Pan-STARRS and later the LSST will flood the community with really huge number of optical sources and by repeating observations at a high rate, will open the possibility to explore the variability time window. In the X-ray domain, XMM-Newton operations may be extended for a decade or more. Also, the German-Russian project eRosita7, to be launched in 2013, will map the entire sky.

We will therefore extensively document the software tools and associated algorithms in scientific and technical publications. We will also train a number of software engineers and astronomers to use these tools so that the work investment proposed here becomes sustainable.

Goal 3: Product distribution

We will make available to the international community the cross-correlated catalogues, the resulting SEDs and all relevant documentation. These data will be hosted at ESAC and/or at the CDS and will be accessible through VO compliant services. The documentation will consist in technical reports and in articles published in refereed journals. Scientific results arising from our test science cases will also help to understand the nature and potential of these new products. Part or all of the software tools developed during this project will be publicly available and offered as stand-alone software packages and as dedicated services at CDS (e.g. theprobabilistic cross-matching tool).